The present invention relates generally to vibratory hammers having at least one pair of laterally spaced, rotating eccentric weights and a drive mechanism for driving rotation of the weights at the same speed in opposite directions to one another. More particularly, the invention relates to a vibratory hammer construction in which the weights can be shifted from their eccentric, vibration-producing positions to balanced, non-eccentric positions during start-up and shut-down to avoid the production of potentially destructive strokes that might otherwise result and to be able to control the magnitude of forces generated by the rotation of the eccentric weights.
A known vibratory hammer construction includes a support frame adapted to be suspended from a crane or the like, a vibrator assembly suspended from the frame for generating vibration used to drive a work piece into the earth, and a clamping assembly for selectively clamping and releasing a work piece. The vibrator assembly includes at least one pair of laterally spaced, parallel shafts that are rotatable in opposite directions to one another, and an eccentric weight supported on each rotary shaft such that the center of mass of the weight is radially offset from the longitudinal axis of the corresponding shaft.
A hydraulic or electric motor is provided for driving rotation of the shafts at a speed of, e.g. 1500 rpm to produce vibration in the hammer for driving the work piece into the earth. At start-up, the motor increases the rotational speed of the shafts from zero rpm to the working speed of the hammer. Likewise, when the hammer is shut down, the speed of the shafts is reduced to a stop. A common problem experienced with known hammers arises during these start-up and shut-down periods. As the hammer vibrates at a frequency equal to its natural frequency, potentially destructive strokes can occur that can cause damage to the hammer, the work piece, and/or possibly to adjacent structures such as buildings and the like. Although operators attempt to bring the hammers up to speed or shut them down as quickly as possible to minimize the amount of damaging vibration that occurs. This practice does not eliminate the risk.